Soil Chemistry

Broadbalk soil chemical properties background information

The Broadbalk wheat experiment, established in the autumn of 1843, has been cultivated since at least 1623, and probably much earlier (Avery & Bullock, 1969). Soil chemical properties have been measured at regular intervals since 1865.

Soil pH: The plough layer (0-23 m) is limed when necessary to maintain a minimum soil pH of 7.0 7.5. Broadbalk was first limed regularly from 1955-1967, with plots given the larger inputs of ammonium fertilizers receiving more lime than the controls. Liming stopped from 1968-1975, but began again in 1976-1992, with 3 or 4 sections being limed each year. From 2007 onward selected plots have been limed every 5-6 years based on soil pH measurements to maintain top-soil pH around 7.0-7.5. Selected plots were limed in autumn 2018.

Data available

Soil chemical properties have been measured at regular intervals on Broadbalk since 1865, in topsoils (0-23cm) and subsoils. See soil measurements 1843-1944 and soil measurements 1966-2017 for details of what data is available. Not all plots, sections or soil depths have been measured every year. The following soil chemical properties have been measured:

Soil Sampling

Broadbalk soil has been sampled on many occasions over the years. However, because the method of sampling has changed and the experiment has been divided, first into two halves, then into five sections and finally into 10 sections, it is not always advisable to directly compare one sampling with another without careful thought. This table shows, where possible, how samples taken over time relate to each other. Selected plots and depths have been sampled on other occasions; most samples still exist in the Sample Archive.

Since 1992 a systemic sampling plan has been adopted. In 1992, 1997, 2005, 2010 and 2015 all five continuous wheat sections were sampled (0, 1, 6, 8 and 9). In the intervening years the remaining sections in rotation were sampled, one per year, so that all sections were sampled every five years. All sections were sampled in 2000 prior to treatment changes being introduced. See table for full details.

In autumn 2004 it was apparent that parts of the field had been ploughed slightly deeper than 23cm as sub-soil clay was visible in random patches across the field. Thus in 2005 and 2006 all plots were resampled to create a new baseline, if necessary, for soil chemical properties. The sections were then sampled systematically from 2008 onwards.

In 2000-2004 archived soil samples from selected plots sampled in 1865, 1881, 1893, 1914, 1936 and 1944 were re-analysed for soil pH, Olsen-P, exchangeable cations, Total %N, %SOC and CaCO3-C. Values for %N and %SOC from the re-analysis of the 1865 samples were very different to the original data and the 1881 and 1893 re-analysed soils, so the original data was used.

Data before 1926 (when the experiment was divided into sections) are given for whole plots only, from the re-analysis in 2000/2004. More recent data are available for individual plots within each section.

1936, 1944 and 1966 data is included with later data, although the experiment was not divided into 10 sections until 1968.

1936 data: 10-20 cores taken with a narrow auger from each of the five Old Sections were bulked within each plot. Data for Old Section I is used for current sections 0 and 1, Old Section II for current sections 2 and 3, etc. (see soil sampling plan for more details). All the data is from the re-analysis of old samples in 2001-2002.

1944 data: four holes were taken from each of the (then) five Old Sections; holes 1 & 2 were on current Section 0, holes 3 & 4 on current Section 1 etc, so the data can confidently be allocated to the modern sections. Data is presented as the mean of the two holes for the topsoil. Subsoil data is not available for each modern section, as the samples were bulked from four-sub-samples for each of the five old sections. Data for Old Section I is used for current sections 0 and 1, Old Section II for Sections 2 and 3, etc. (see soil sampling plan for more details). The soil was re-analysed in 2003-5 from selected plots from all Sections and depths. All data is from the re-analysis except %CaCO3 which was analysed in 1944. Soil pH and Exchangeable Na were not measured in the 1944 subsoil samples.

1966 data: Section 0 and 1 use data from Old Sections Ia and Ib respectively; Sections 2 and 3 use data from Old Section II; Sections 4 and 5 use data from Old Section III; Sections 6 & 7 use data from Old Section IV; Sections 8 and 9 use data from Old Sections Va and Vb respectively. Bulked Va and Vb samples (ie Sections 8 and 9) were used to determine %SOC (plots 2.1 and 2.2 only) and for all plots for Olsen P, %CaCO3 and Exchangeable K. There was no plot 1 in 1966, this was created in 1968. The soil was sampled in September 1966 but no day is given. It is shown as 15/09/1966 in the database.

Data from the samplings in 2001-2004 is not included as this only covered sections 2, 4, 5 and 7 and there was a comprehensive sampling of all sections in 2000 and then in 2005/6.

Soil Sampling Methods

Samples between 1865 and 1914 were taken with an open-ended metal box, 9 inches (23cm) deep and usually 6 x 6 inches (15 x 15cm) across. There were between three and eight sample positions on each plot which were bulked together for each depth on each plot. In 1944 a spade was used to sample the 0-23cm layer, and the subsoil (23-46cm) was sampled with a semi-cylindrical auger.

In 1936 and from 1966 onwards samples were taken with a semi-cylindrical auger. 10-20 cores were taken from the different soil layers for each individual plot within each section and bulked together for each plot. Small diameter cores taken by semi-cylindrical augers cannot be used to determine soil weights, but provided enough are taken the sample better represents the proportions of SOC, N, P, K etc in the soil than a few large box samples. See soil physical properties for details of soil weights.

Samples were taken in the autumn, after the crop had been removed, but before ploughing, except for Section 3, 1996 which was sampled in March.

Broadbalk sampling
Soil sampling Broadbalk 1943

Non-herbicide plot section 8 Broadbalkp
Soil sampling Broadbalk 1943

Broadbalk sampling
Broadbalk ploughing in 2013

Analysis methods for soil chemical properties

All soil samples are air-dried and sieved <2mm. Data are given for air-dried soil (approximately 98% dry matter). When calculating total amounts in the soil (e.g. kg N ha-1 ) you may wish to convert to oven-dry soil (i.e. 100% dry matter). Broadbalk standard soil weights are given for oven-dry soil.

For information on current analytical methods used for the Rothamsted Long-term experiments, please contact the Rothamsted Research Analytical Chemistry Unit - Harpenden laboratory,

Total soil % nitrogen (%N)

1865: Original soda lime analysis for total N (Johnston, 1969b, table 5.10) multiplied by a factor derived from the comparison of soda lime and LECO analysis values for 1881 and 1893 samples. Soda lime analysis by the method of Will and Varrentrapp (Watt, 1863). See Johnston (1969a, p 50) for more details.

1881-1944: Selected samples re-analysed in 2001-4 by combustion analysis, based on the Dumas method, using a LECO combustion system. Measured on air-dried, finely ground soil (to pass a 355 micron or 44 mesh sieve).

1966, 1987-8: Kjeldahl digest method for total N (Bremner, 1965). The digest was then analysed colorimetrically using a Technicon continuous flow analyser. Measured on air-dried soil, finely ground to pass a 0.5mm sieve.

1992 onwards: Combustion analysis, based on the Dumas method, using a LECO combustion system. Measured on air-dried, finely ground soil (to pass a 355 micron or 44 mesh sieve).

Soil % organic carbon (%SOC)

1865: Derived from original soda lime analysis for total N and C:N ratios for 1893 for organic carbon (Dyer, 1902).

1881-1944: Selected samples re-analysed in 2001-4 by combustion analysis, based on the Dumas method, using a LECO combustion system to measure total carbon. SOC determined as total C minus CaCO3-C, measured by a calcimeter (see below). Measured on air-dried, finely ground soil (to pass a 355 micron or 44 mesh sieve).

1966: Chromic acid titration method (Walkley and Black, 1934). Correction factor of W-B x 1.3 used, which is equivalent to organic C by Tinsley or total C by combustion minus CaCO3-C. But see also Johnston (1969b, p 97). Measured on air-dried soil, ground to pass a 0.5mm sieve.

1987-8: Dichromate digestion, modified Tinsley (Kalembasa and Jenkinson, 1973) to measure organic C. Measured on air-dried soil, finely ground to pass a 0.5mm sieve.

1992 onwards: Combustion analysis, based on the Dumas method, using a LECO combustion system to measure total C. Measured on air-dried, finely ground soil (to pass a 355 micron or 44 mesh sieve). SOC determined by subtraction of CaCO3-C, measured by a calcimeter (see below).

Inorganic carbon (IC) also known as calcium carbonate-C or CaCO3-C

Soils sampled in 1865-1936 were re-analysed in 2000-4. Most of the 1944 data was from the 1944 analysis, except for a few plots (9, 18 and 19) that were analysed in 2001. IC was not measured in 1987-88, as %SOC was determined directly by Tinsley analysis. In other years IC is subtracted from total carbon to give %SOC.

All samples up to 2012 were analysed by a calcimeter. CO2 is liberated from CaCO3 in the soil sample by treating with hydrochloric acid (HCl) in a closed system. The amount of CaCO3 is calculated by comparing the pressure produced by the sample against the pressure produced by known weights of CaCO3, using a mercury filled manometer. %CaCO3-C or %IC is derived from %CaCO3 by dividing by 8.333. Since 2014 inorganic C has been measured by an automated Skalar Primacs inorganic carbon analyser.

Soil pH

Soil pH in water, with a 1:2.5 soil:water suspension, mean of two readings. Soils from selected treatments sampled in 1865-1944 were re-analysed in 2000-4. Measured on air-dried soil, sieved < 2mm.

Olsen-P (plant-available phosphorus; also known as bicarbonate soluble-P or NaHCO3-soluble P)

Soils from selected treatments sampled in 1865-1944 were re-analysed in 2000-4. All samples were analysed by the Olsen method, in which soil is extracted with a solution of 0.5M NaHCO3, buffered at pH 8.5 (Olsen et al, 1954). The extract is then analysed by continuous segmented colorimetric flow analysis. Measured on air-dried soil, sieved < 2mm.

See Blake et al (2000, 2003) for a discussion of the P balance on Broadbalk and changes in soil P fractions over time (Key References, below).

Exchangeable cations - Calcium (Ca), magnesium (Mg), potassium (K) and sodium (Na)

Selected soils sampled in 1865-1944 were re-analysed in 2000-6, and those sampled in 1966 were re-analysed in 2019. All soils were extracted with 1M ammonium acetate (NH4CH3CO2) solution, after the method of Metson (1956). The data is expressed as mg kg-1 in dry matter, with 5mg of soil in 100ml of leachate. Since 1983, the extracts have been analysed by ICP-OES (Inductively Coupled Plasma - Optical Emission Spectrometer). Measured on air-dried soil, sieved < 2mm.

Blank (control) values: Analysis blanks were subtracted from exchangeable Na, and other cations if necessary (e.g. K in 1987).

Samples taken in 1966 were re-analysed in 2019. The exchangeable K values differ from those published by Johnson (1969b), when a slightly different technique was used for measuring exchangeable cations, which involved swirling the samples in successive amounts of ammonium acetate and decanting off the solution once settled. In 1966 the extracts were measured with a flame photometer. Bolton (1972) reports on exchangeable Ca and Mg in Broadbalk soils from 1856-1966 for some sections, from the western (top) end of the field, but the 1966 data has not been found. It is recommended that the re-analysed data from 2019 is used for the 1966 samples.

See Bolton (1972) for changes in exchangeable Ca and Mg, 1856-1966 and Blake et al (1999) for a study of soil K content, crop K uptake and K balance in Broadbalk (Key References, below).

Methods References:

  • Bremner, J. M. (1965). Total nitrogen. In: Methods of Soil Analysis. Part 2 (ed. C. A. Black), pp. 1149-1178. Madison: American Society of Agronomy
  • Johnston, A. E. (1969a) "The plant nutrients in crops grown on Broadbalk", Rothamsted Experimental Station Report for 1968, Part 2, 50-62 DOI:10.23637/ERADOC-1-34916
  • Johnston, A. E. (1969b) "The soils of Broadbalk: Plant nutrients in Broadbalk soils", Rothamsted Experimental Station Report for 1968, Part 2, 93-115 DOI:10.23637/ERADOC-1-34923
  • Kalembasa, S.J and Jenkinson, D.S (1973) A comparative study of titrimetric and gravimetric methods for the determination of organic carbon in soil. Journal of the Science of Food and Agriculture 24: 1085-1090.
  • Metson, A.J. (1956). Methods of chemical analysis for soil survey samples. New Zealand Soil Bureau, Bulletin 12.
  • Olsen S.R., Cole C.V., Watanabe F.S., Dean L.A. (1954) Estimation of available phosphorus in soils by extraction with sodium bicarbonate. USDA Circular 939, US Gov. Print. Office, Washington, D.C.
  • Walkley, A. and Black, I.A. (1934). An examination of the Degtjareff method for determining soil organic matter and a proposed modification of the chromic acid titration method. Soil Science 37: 29-38.
  • Watt, H. (1863) A dictionary of chemistry, vols. 1-4. London: Longman, Green, Longman, Roberts and Green.
  • Further information and acknowledgements

    See Goulding et al (2000) for measurements of nitrate leaching from the Broadbalk wheat experiment from 1990-1999 (Key References below).

    With thanks to Andy Macdonald, Paul Poulton and Steve Freeman for help with compiling the data and text.

    For further information and assistance, please contact the e-RA curators, Sarah Perryman and Margaret Glendining using the e-RA email address: era@rothamsted.ac.uk